1. Field of the Invention
The subject invention relates to dynamic valve seals, and more particularly, to a dynamic seal arrangement in valve assemblies employed in the fuel injectors of gas turbine engines.
2. Description of Related Art
Check valves and schedule valves are commonly used in the fuel injectors of gas turbine engines to control fuel flow. These valves are designed to prevent the fuel system from draining on engine shutdown. They also eliminate fuel leaking into combustor on shutdown, provide consistent system pressures during operation, and contain fuel expansion, driven by elevated fuel temperatures during soak back, after engine shutdown.
Drip tight seals are required in many check valves and schedule valves (also referred to as metering valves) used in fuel injectors. Typically, drip tight seals have been made from elastomers. For example, FKM (fluoroelastomer, i.e. Viton) or FFKM (perfluoroelastomer, i.e. Kalrez) have been used when engine operating conditions reach temperatures above 350° F. Elastomers are commonly used as o-rings or as molded seals attached to metallic components.
The elastomer used for a valve seal is typically the material which limits the allowable operating temperature of the valve. High temperatures degrade the elastomer (faster aging with higher temperature). Degraded elastomers tend to stick to metal and take on a permanent set. Cold temperature shrink the elastomer and prevent it from forming a tight seal.
In most prior art check valves and schedule valve, the sealing surfaces move away from the seal as the valve opens. The elastomer seal is typically trapped in or molded to one of the valve components to prevent it from moving out of position. When the valve closes, the seal surface moves back to the elastomer and the leak proof seal is re-formed.
Spring energized PTFE seals are also used in fuel injectors. In such instances, they are typically positioned to allow system pressure to expand the inside of the seal. Moreover, the internal pressure on the PTFE would open the seal as the valve opens and possibly deform/extrude the PTFE seal. The normal position of the seal may work for relatively low pressure valves, but an extrusion failure mode may always be possible. Given these factors, using a spring energized PTFE seal in its normal orientation, as a dynamic valve seal would be problematic.
The solution to this problem, as provided by the subject invention, is to use the spring energized PTFE seal in a backwards orientation, facing the internal cavity of the seal towards the downstream pressure side of the valve assembly. This prevents the internal pressure from deforming the PTFE. By replacing a typical elastomeric o-ring seal with a spring energize PTFE seal, the temperature capability of the valve assembly can be greatly increased. Moreover, the likelihood of a thermal permanent set on a PTFE seal when compared to an elastomeric seal is reduced significantly due to the hardness of the seal material.